15
20 In a nuclear reactor, the area of a square metal sheet with sides x decreases at a rate of 5
cm
2
s
-1
when it is cooled. Find the rate of change of x, in cm s
-1
, at the instant when
x = 10 cm. [3 marks]

16
22 4 students are chosen to represent the school in the International Mathematical
Olympiad. These students are chosen from 6 boys and 5 girls. Calculate the number of
different ways the students can be chosen if